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Hunting for jet quenching in collisions between oxygen nuclei

8 September 2025 | By

In summer of 2025, the ATLAS Experiment recorded its first-ever oxygen–oxygen collisions, opening a new window onto the study of quark-gluon plasma (QGP). This extreme state of matter mimics the conditions of the early Universe during the first microseconds after the Big Bang. Traditionally, researchers have focused on studying QGP formed in the collisions of heavy ions (like lead or xenon), which maximize the size of the plasma droplet created. But in recent years, interest has grown in exploring the QGP using smaller ions (such as oxygen) to better understand the QGP across different system sizes (see Figure 1).

The ATLAS Collaboration has just released its first search for jet quenching in oxygen–oxygen collisions. This phenomenon occurs when high-energy particle jets lose energy as they traverse the QGP. Physicists use these jets rather like a dipstick, probing the nature and properties of the QGP.

Plots or Distributions,Physics,ATLAS — Figure 1: Transverse images of the size and shape of typical collision regions for lead-lead, xenon-xenon, oxygen-oxygen, and proton-lead collisions (from left to right). In each case the filled circles represent the region where the QGP will be formed and the shaded circles show protons and neutrons in the nuclei which miss the oncoming nucleus and do not participate in the collision. (Image: ATLAS Collaboration/CERN)

The ATLAS Collaboration has measured jet quenching in oxygen–oxygen collisions – the smallest system yet to show this phenomenon.

Lead–lead and xenon–xenon collisions form large plasma droplets, allowing jets to travel long distances through the QGP. In both systems, the ATLAS Collaboration has observed clear signs of jet quenching. By contrast, proton–lead collisions produce tiny droplets of QGP, and no evidence of jet quenching was found. Oxygen–oxygen collisions offer a middle ground. Could jet quenching occur in this smaller system? If so, what would it look like?

To answer these questions, ATLAS physicists turned to one of the most sensitive tools for studying the QGP: dijet momentum balance. In proton-proton collisions, pairs of jets (dijet) are typically produced back-to-back with nearly equal momentum. But in lead-lead collisions, an increase in imbalanced dijets has been observed, arising either from differences in the jet’s path length through the QGP (see Figure 2) or from fluctuations in the energy-loss process itself. Researchers quantify this imbalance using the ratio (x_J), which is the ratio of the lower jet transverse momentum to the higher one. In head-on (central) lead-lead collisions, they found a strong reduction in balanced jets (with x_J~1) and an increase in imbalanced jets (with x_J~0.5-0.6) when compared to proton–proton collisions.

In their new result, ATLAS physicists found the same trend in central oxygen-oxygen collisions – but at a smaller scale. Instead of a nearly 50% reduction in the number of balanced dijets (those with x_J~1, as seen with lead) the reduction is much smaller (see Figure 3). This result marks the first measurement of jet quenching in oxygen–oxygen collisions, and the smallest system scale in which this phenomenon has been observed.

This milestone, achieved just nine weeks after data-taking, reflects a remarkable effort made by members across the ATLAS Collaboration. The result provides a vital new piece in the jet-quenching puzzle, helping researchers pinpoint the system size at which QGP begins to affect high-energy jets. Future studies of the oxygen-oxygen dataset are set to explore this phenomenon in greater detail, offering new insights into this exotic state of matter.

About the event display: Collision event recorded by the ATLAS experiment on 5 July 2025, when stable beams of oxygen, colliding at a centre-of-mass energy per nucleon pair of 5.36 TeV, were delivered to ATLAS by the LHC. (Image: ATLAS Collaboration/CERN)